Typically a fast transient occurs at the output of an audio amplifier when the output is enabled, e.g., upon power-up. Sometimes an audio transducer, such as a speaker or headphone, is coupled to the amplifier output during power-up or enablement. The fast transient is then audible as a "click" or "pop." Referring to FIG. 1A, an amplifier or driver 100, such as an op-amp, receives an audio signal via a lead 105. If amplifier 100 is used in a low voltage application, such as an integrated circuit, then the audio signal amplitude can vary in a range of between zero and five volts, inclusive. Such an audio signal usually has a common mode voltage between the limits of the voltage range. For example, the common mode voltage can be 2.5 volts.
When amplifier 100 is powered-up or its output is enabled while receiving the audio signal, a fast transient will occur on an output lead 110. Specifically, an enable or power-up signal is illustrated in FIG. 1B. The signal is inactive until a time T1, at which point amplifier 100 in FIG. 1A is powered-up or the output enabled. An output signal from amplifier 100 is provided on lead 110. This output signal, as shown in FIG. 1C, has a transient that occurs between times T2 and T3. Since lead 110 is coupled to an audio transducer, this transient output signal on lead 110 is audible as a pop or click. Portion 115 of the output signal shown in FIG. 1C represents the common mode voltage of the audio signal input to amplifier 100. Portion 115 can have an audio signal superimposed thereon.
To suppress this audible click or pop, many designs are utilized. One such design uses a feedback loop with the amplifier. Another design uses muting circuitry that essentially times turning the amplifier on and off. A further design controls the output stage of the amplifier. These implementations are complex and waste integrated circuit area.
A need exists for an implementation that eliminates the audible click or pop while being simple in design and using minimal integrated circuit area. The present invention meets this need.